The diesters of naphthalenedicarboxylic acids are useful for preparing a variety of polymeric materials such as polyesters and polyamides. One particularly useful diester is dimethyl-2,6-naphthalenedicarboxylate (DM-2,6-NDC). Dimethyl-2,6-naphthalenedicarboxylate, for example, can be condensed with ethylene glycol to form poly(ethylene-2,6-naphthalate) (PEN), a high performance polyester material. Fibers and films made from PEN have considerably improved strength and superior thermal properties relative to, for example, poly(ethyleneterephthalate). For this reason, PEN is an exceptional material for preparing commercial articles such as thin films which can be used in the manufacture of magnetic recording tape and electronic components. Additionally, because of its superior resistance to gas diffusion, and particularly to the diffusion of carbon dioxide, oxygen and water vapor, films made from PEN are useful for manufacturing food containers, especially the so-called "hot fill" food containers. PEN can also be used to prepare high strength fibers useful for the manufacture of, for example, tire cord.
DM-2,6-NDC is most readily prepared by the esterification of 2,6-naphthalenedicarboxylic acid (2,6-NDA) with methanol. The 2,6-NDA is conveniently prepared by the liquid phase, heavy metal catalyzed oxidation of a 2,6-dialkyl- or 2-alkyl-6-acyl-naphthalene compound using molecular oxygen as the source of oxygen for the oxidation reaction. During this oxidation reaction, impurities such as 2-formyl-6-naphthoic acid (FNA), trimellitic acid (TMLA) and, when bromine oxidation promoter is used, various brominated compounds are produced. Although in some instances it would be desirable to use 2,6-NDA directly for the preparation of PEN, however, because of its high melting point (&gt;300.degree. C. with decomposition) and extremely low solubility in ordinary solvents, 2,6-NDA is difficult to purify to acceptable levels by standard purification techniques such as adsorption or recrystallization. These difficulties in purifying 2,6-NDA are partially overcome by converting 2,6-NDA to its dimethyl ester, DM-2,6-NDC. DM-2,6-NDC can be distilled and it can be recrystallized from solvents such as methanol or from one or more aromatic solvents.
For a large-scale commercial process for preparing diesters of naphthalenedicarboxylic acids, such as dimethyl-2,6-naphthalenedicarboxylate, it would be highly advantageous to produce the diester in a continuous manner at high reaction rates, using a minimum amount of alcohol component for the esterification reaction, as well as using the least expensive reactor equipment. There is a continuing need for improved processes for esterifying naphthalenedicarboxylic acids, such as 2,6-naphthalenedicarboxylic acid. The present invention provides such an improved process.
Methods for preparing diesters of naphthalenedicarboxylic acids are known. In a typical process, a naphthalenedicarboxylic acid, such as 2,6-naphthalenedicarboxylic acid, is esterified with methanol at 120.degree.-220.degree. C. using sulfuric acid catalyst. A higher-temperature process is disclosed in U.S. Pat. No. 4,003,948 to Yamashita et al., wherein 2,6-naphthalenedicarboxylic acid is continuously esterified with methanol, in the presence or absence of an esterification catalyst, wherein the 2,6-naphthalenedicarboxylic acid is fed into an esterification reaction zone in the form of solid particles having a specific surface area of about 4,000 cm.sup.2 /g. For continuous operation, it is disclosed therein that a plate tower-type or a stirred vessel-type esterification reaction can be used.
Japanese Patent Application No. 50-83362 (1975) discloses a method for continuously esterifying 2,6-naphthalenedicarboxylic acid by reacting 2,6-naphthalenedicarboxylic acid with methanol while the methanol concentration in the liquid phase of the esterification reaction container is 2-20 wt. %. In the Japanese Patent Application No. 50-83362 (1975), it is disclosed that it is desirable to feed a suspension of 2,6-naphthalenedicarboxylic acid in dimethyl-2,6-naphthalenedicarboxylate to the uppermost step of a step-type column reaction container, and that methanol be fed at a position lower than the lowermost step of the step-type column. The example provided in the Japanese Patent publication describes such a counter-current addition of the reactants to the step-type reactor, and where the 2,6-naphthalenedicarboxylic acid is slurried with four times its weight of dimethyl-2,6-naphthalenedicarboxylate before it is added to the reactor. Such a process, therefore, requires a large recycle of the product to the esterification reactor. Processes for esterifying 2,6-naphthalenedicarboxylic acid are also disclosed in Japanese Patent Applications Nos. 48-96574 (1978), 50-76055 (1975), 50-76057 (1975), and 50-95253 (1975). Finally, prior processes for preparing dimethylterephthalate from terephthalic acid have used a two-compartment stirred tank reactor in combination with a plug-flow reactor, under essentially liquid-fill conditions and long residence times of about 1 hr, and at reaction temperatures of about 260.degree. C.